Method and circuit for transferring data with dynamic parity generation and checking scheme in multi-port DRAM

ABSTRACT

An ATM switch including a multi-port memory is described. The multi-port memory having a dynamic random access memory (DRAM) and a plurality of input and output serial access memories (SAMs). Efficient, flexible transfer circuits and methods are described for transferring ATM data between the SAMs and the DRAM. The transfer circuits and methods include helper flip/flops to latch ATM data for editing prior to storage in the DRAM. Editing of ATM data transferred from the DRAM is also described. Dynamic parity generation and checking is described to detect errors induced during switching.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to ATM switches and inparticular the present invention relates to internally transferring datain a multi-port memory ATM switch.

BACKGROUND OF THE INVENTION

The general increase in network traffic requires fast, efficient methodsof managing traffic and congestion. One problem area in network trafficmanagement occurs at line switching where a switch is used to route datafrom one set of communication lines to another set of communicationlines. Network switches typically comprise a memory-type device which isused to temporarily store a transmission during the switching operation.

Different queuing structures have been employed in network switches withvarying results. It has been shown that switches which use input queuestructures tend to create line blocking. Conversely, switches which useoutput queue architectures eliminate the line blocking problem. Further,switches used in asynchronous transfer mode (ATM) networks require morestorage space than switches used in a more uniform transmission network.That is, ATM traffic tends to have bursts of data and be non-uniform,thereby requiring an increased amount of available memory at any giventime to maintain an acceptable switching time for the ATM traffic. Poorswitch time performance can result in the loss of ATM data duringswitching operations.

The increased demand for more memory and the need for faster switchingtimes has resulted in the need for a fast, efficient ATM switch.Further, the variety of different ATM data cell structures requires thatthe switch be flexible. One specific problem in ATM switching is theneed to change the routing of an ATM cell to avoid highly congestedtraffic paths. This typically requires that a header included in the ATMcell be amended to denote a new route, or destination address.Additional information included in an ATM cell, such as error checkdata, may also need to be updated. Further, because memories aresusceptible to the inducement of errors, an ATM switch should include anerror check scheme to monitor the occurrence of any errors inducedduring switching.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art fora fast, flexible ATM switch which efficiently updates the muting ordestination address of an ATM cell. There is a further need for such aswitch which includes an internal error check.

SUMMARY OF THE INVENTION

The above mentioned problems with ATM switches and other problems areaddressed by the present invention and which will be understood byreading and studying the following specification. A transfer circuit andmethod are described in a multi-port memory which provide a fast,flexible ATM switch.

In particular, the present invention describes a multi-port memorycomprising a plurality of input serial access memories (ISAMs) used tostore asynchronous transfer mode (ATM) data, a dynamic random accessmemory (DRAM) used to receive and store the ATM data from the ISAMs, anda plurality of output serial access memories (OSAMs) used to receive andstore the ATM data from the dynamic random access memory. The multi-portmemory also comprises an error code generator, and a write transfercircuit connecting the ISAMs, the error code generator, and the DRAMtogether. The error code generator in the preferred embodiment is aparity code generator. The write transfer circuit includes an editbuffer, an error code check circuit, and a read transfer circuitconnecting the error code check circuit, the OSAMs, and the dynamicrandom access memory together. The read transfer circuit includes anedit register.

The write transfer circuit can further comprise a plurality of writetransfer buses having one bus electrically coupled to one bit of eachISAM. The error code generator circuit comprises a plurality of helperflip/flops with each one of the plurality of helper flip/flops connectedto one of the plurality of write transfer buses, and an error codegenerator connected to the plurality of helper flip/flops and generatingan error signal therefrom. Further, the edit buffer can comprise aplurality of helper flip/flops with each one of the plurality of helperflip/flops connected to one of the plurality of write transfer buses.

In another embodiment, the read transfer circuit comprises a pluralityof read transfer buses having one bus electrically coupled to one bit ofthe plurality of output serial access memories. Further, the error codecheck circuit can comprise a plurality of helper flip/flops with eachone of the plurality of helper flip/flops connected to one of theplurality of read transfer buses, an error code generator connected tothe plurality of helper flip/flops and generating an error codetherefrom, and a comparator connected to the error code generator whichcompares the generated error code with a pre-defined error code.Finally, a parity error circuit can be included to monitor the errorcode check circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a multi-port memory;

FIG. 2 is a detailed block diagram of the multi-port memory of FIG. 1;

FIG. 3 is a 56 byte ATM cell having a specific byte assignment;

FIG. 4 is a 52 byte ATM cell having a specific byte assignment;

FIG. 5 is a 53 byte ATM cell having a specific byte assignment;

FIG. 6 is a block diagram of the transfer circuit of the multi-portmemory of FIG. 2;

FIG. 7 is a schematic of a helper flip/flop portion of the transfercircuit of FIG. 6; and

FIG. 8 is a graph of a transfer of data from an input serial accessmemory to an edit buffer circuit of the multi-port memory of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiment,reference is made to the accompanying drawings which form a part hereof,and in which is shown by way of illustration specific preferredembodiments in which the inventions may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that logical, mechanical and electricalchanges may be made without departing from the spirit and scope of thepresent inventions. The following detailed description is, therefore,not to be taken in a limiting sense, and the scope of the presentinventions is defined only by the appended claims.

The present invention is described as being incorporated in a multi-portmemory. The simplified block diagram of the multi-port memory 100 shownin FIG. 1 can be used to solve data path requirements for asynchronoustransfer mode (ATM) networks. The memory has a dynamic random accessmemory (DRAM) 102 and eight double-buffered input serial access memories(SAMs) 104(0)-(7). The eight output SAMs 106(0)-(7) are alsodouble-buffered. The input SAM's can accept back-to-back ATM cells suchthat one full ATM cell can be moved into the DRAM from an input portwhile a second ATM cell is being loaded into the input port. Likewise,the output SAM's can output one ATM cell on a four bit data bus while asecond ATM cell is being transferred from the DRAM.

ATM cells flow through input ports into the input SAM's where editingcan be performed. The ATM cells are transferred to the DRAM and thentransferred to the output SAM's where further editing can be performed.The ATM cells are output on communication lines via output ports.

A more detailed diagram of the multi-port memory 100 is shown in FIG. 2.The memory has a DRAM 102, input SAMs 104(0)-(7) and output SAMs106(0)-(7) as described above. Each input port has a correspondingbuffer circuit 108. The buffer circuits 108 are used to perform severalfunctions including latch and buffer input data, queue nibble counter,and pointer decoder and control. Four-bit input data communication buslines 110 provide ATM cell input to the buffer circuits 108. An inputframe and clock circuit 112 connected to the buffer circuits bothdenotes the beginning of an input cell and synchronizes the input data.

Each output SAM 106 has a corresponding buffer circuit 116. The buffercircuit 116 is used to perform several functions including latch andbuffer output data, queue nibble counter, and pointer decoder andcontrol. Four-bit output communication bus lines 118 provide ATM celloutput from buffer circuits 116. An output frame and clock circuit 120connected to the output buffer circuits denote the beginning of anoutput cell and synchronizes the output data.

Byte parity generator 122 provides a parity code for each ATM celltransferred to the DRAM from an input SAM 104, and byte parity check 124checks the parity of an ATM cell transferred from the DRAM to an outputSAM 106. The byte parity generator 122 and byte parity check 124 aredescribed below.

Control interface and command decode 114 interfaces the multi-portmemory 100 with an external controller (not shown) and decodes commandsprovided on interface lines 206 therefrom. Error/status circuit 126monitors the status of the input SAM's 104 to determine if a transfer tothe DRAM is needed and tracks the output from the byte parity check 124to determine if an error occurred in an ATM transfer. Cyclicalredundancy check (CRC) logic 128 and edit buffer control 130 togetherprovide a means of modifying the ATM cell prior to transferring it froman input SAM port 104 to the DRAM, as described below. An output editor132 and output edit buffer 134 allow for the modification of the ATMcell prior to outputting through the output SAMs 106. Memory controlcircuit 136 provides the circuitry needed to address the DRAM as knownby one skilled in the art.

ATM DATA FLOW

Referring to FIG. 2, in general, ATM cells flow through the buffercircuits 108 into the input serial access memory (ISAM) 104 whereediting can be done by an external control function provided through thecontrol interface. The external control function instructs the chip whento store ISAM cells in the DRAM 102 and when to load the output serialaccess memory ports (OSAM) 106 from the memory 102. The OSAMs deliverthe ATM cells to output buffers 116 for dispatch. The followingparagraphs describe the data flow in more detail.

ATM cells enter the chip through communication line inputs 110 to theinput buffers 108. The 4-bit parallel stream is loaded into an ISAM 104by the clock associated with the buffers over 4-bit data bus 105. Theinput frame and clock circuit 112, marking the first data nibble of theATM cell, is used to start the ISAM load. Once a complete cell has beenloaded into an ISAM, the sam error/status circuit 126 sets a "ISAM full"status bit. As noted above, the ISAMs are two ATM cells long, so thenext ATM cell can continue to stream into the ISAM. An asynchronous,external control function polls the ISAM status through the controlinterface and command decode 114. Once a ISAM full status is detected bythe external controller through the control interface and command decode114, the first 72 bits of the ISAM are copied to the input edit buffer130 and the first or second word of the ISAM can be output to theexternal controller using mux 138. The external controller evaluates andoptionally modifies the word and then initiates a write to the DRAMmoving the content of the input edit buffer (bits 0-71) and theremaining content of the ISAM (bits 72-447) over a write transfer bus tothe DRAM row specified by an address presented through the memory logiccontrol 136. The ISAM full status is cleared when the ISAM istransferred. The first and/or second word of the input edit buffer 130can be returned by the external controller to modify the content of theISAM. Further discussion of the input edit buffer operation is presentedbelow. The entire contents of an ISAM can also be directly transferredto the DRAM without copying or evaluating the first 72 bits. In thisoperation the entire cell is transferred intact.

The external controller polls an output port status through the controlinterface and command decode 114 for empty OSAMs. Once an empty OSAM 106is detected, a DRAM read is initiated to move data at a DRAM rowspecified by an address from the memory control 136 over a read transferbus to the OSAM. The first or second word of the ATM cell can be editedby output edit register 132. Further discussion of the output editregister operation is presented below. Like a write transfer, thetransfer to an OSAM clears the empty OSAM status. OSAMs are doublebuffered, and similar to the ISAMs can continuously stream data out ofthe output buffers 116. The 4-bit parallel stream is clocked over 4-bitdata bus 117 by the clock associated with the output buffers 116 byoutput frame and clock circuit 120.

CELL EDIT OPERATIONS

The multi-port memory stores ATM cells of 56 bytes in the internal DRAM102. The standard ATM cell consists of 4 bytes of header, 48 bytes ofdata, and one byte of header-error-check. The header-error-check istypically a cyclical redundance check (CRC) code, which is used oncommunication links and may or may not be presented to the multi-portmemory. The remaining space, either 3 or 4 bytes depending on thepresence of the CRC, can be used to store specialized routinginformation for space-division switches, specialized error controlinformation, or any other purpose. These additional bytes are referredto herein as "prepend/postpend" data, and the number of bytes of each isconfigurable. The prepend/postpend data can be generated, read, written,and stripped through cell editing operations and configurationparameters as detailed below.

The input edit buffer 130, in concert with the CRC circuitry 128,provides a means to modify the ATM cell captured in an ISAM 104 beforestoring in the DRAM 102. These modifications can be either of the cellheader, or of the prepend/postpend data attached to the cell. Updatingof the cell header can also include an update of the CRC byte.

The output edit register 132 provides the means to modify the ATM cellcontents at the last possible moment prior to outputting the cell. Theoutput edit register provides data to an OR-type logic function,allowing the control interface and command decode 114 to set selectedbits in the cell header or prepend/postpend data. If the cell header isupdated, the corresponding CRC can also be updated using the output editregister.

All ATM cell editing is performed using the external controller. Theexternal controller can get either the prepend/postpend or header wordfrom an ISAM through edit buffer 130 and mux 138. The first 9 bytes (72bits) of the selected ISAM are copied into the input edit buffer 130,and either the first or second word out is passed through mux 138 to thecommand interface and command decode 114, and ultimately to the externalcontroller. A check of the CRC byte can also be performed using CRClogic 128.

After the external controller has examined the header and/or thepre/postpend data, it may update them via a transfer command or atransfer with CRC command. For example, the transfer with CRC commandtells the memory to updated the CRC byte. After updating, the controllerissues a store ISAM command to copy the ISAM data into the DRAM with anymodifications that may have been made in the edit buffer 130 and CRClogic 128.

The output edit register 132 contains 64 data bits that are NOR'ed withthe header and prepend/postpend data bytes from the DRAM on its way toan OSAM. This provides a way for the external controller to modify anATM cell just prior to transmission, to include e.g. up-to-datecongestion information, or up-to-date prepended routing information fora next stage switch. The output edit register 132 is controlled by theexternal controller. Corrections to the CRC byte can also be performedbased on an updated header. The CRC byte is updated by an XOR logicoperation.

Editing support for various cell lengths is accomplished by defining theISAM start and stop positions within the ISAM's 56 byte space. The startand stop pointers are loaded by the command interface and commandcontrol 114, and are set such that the cell header appears in bytes 4-7of the ISAM. The CRC byte, if present, is always in byte 8 of the ISAM.FIG. 3 shows the ISAM space with the ATM cell header properly aligned.For the cases without any pre/postpend data, FIG. 4 show the ISAM startpointer set to 4 and the ISAM stop pointer set to 55 if no CRC ispresent (making a 52 byte cell). Similarly, FIG. 5 shows the startpointer set to 0 if CRC is present (making a 53 byte cell).

There are different configurations for the ISAMs, each giving variousamounts of prepended and postpended data. The values of start and stoppositions for different configurations are given in Table 1. It will beunderstood that values other than these may also be used in a carefullydesigned non-ATM system.

                  TABLE 1                                                         ______________________________________                                                       W/O CRC       With CRC                                                        Start                                                                              Stop     Start  Stop                                      ______________________________________                                        Without pre/postpend data                                                                      4      55       4    0                                       1 byte prepend   3      55       3    0                                       1 byte postpend  4      0        4    1                                       2 bytes prepend  2      55       2    0                                       1 byte prepend & 1 byte                                                                        3      0        3    1                                       postpend                                                                      2 bytes postpend 4      1        4    2                                       3 bytes prepend  1      55       1    0                                       2 bytes prepend & 1 byte                                                                       2      0        2    1                                       postpend                                                                      1 byte prepend & 2 bytes                                                                       3      1        3    2                                       postpend                                                                      3 bytes postpend 4      2        4    3                                       4 bytes postpend 0      55                                                    3 bytes prepend & 1 byte                                                                       1      0                                                     postpend                                                                      2 bytes prepend & 2 bytes                                                                      2      1                                                     postpend                                                                      1 byte prepend & 3 bytes                                                                       3      2                                                     postpend                                                                      4 bytes postpend 4      3                                                     ______________________________________                                    

The OSAM start and stop address are separately configurable from theISAM ones. Thus, outgoing cells can have more bytes or fewer bytes thanincoming cells. This feature provides the capability for adding ordeleting prepended and postpended data bytes. The table of start andstop addresses is identical to that for the ISAMs given above in Table1.

TRANSFER OPERATIONS

As described above, there are 376 data bus lines and 72 other bus lines(56 bytes) connecting the ISAMs to the byte parity generator 122 througha write transfer bus. After the parity bytes are generated by byteparity generator 122, there are 504 bus lines (63 bytes) connected tothe DRAM. Similarly, 504 bus lines connect the DRAM to the byte paritycheck 124 and a total of 448 lines are connected to the OSAMs through aread transfer bus. Four of these bus lines are illustrated in FIG. 6,two data buses and two header bus lines. Transfer buses 140(0) and (1)are the first two buses used to connect the first two bits of theprepend/postpend byte of the ATM cell from ISAMs 104 to DRAM 102.Transfer buses 142(446) and (447) are the last two bits of the 56 bytesof the ATM cell. FIG. 6, shows the "true" transfer bus line. It is notedthat for each transfer bus there is an accompanying complement transferbus line which is not illustrated.

In the most simple example, the contents of an ISAM 104 are writetransferred to the DRAM 102 and then read transferred to an OSAM 106.Accurately and efficiently transferring an ATM cell, however, oftenrequires manipulation of portions of the cell as described above.Transfer buses 140(0) and (1) can be connected through transistors 144to the first two bits of each ISAM, noted as 104(0)-104(N) (where n isequal to 7 in this embodiment). Control lines 146 can be selectivelyactivated to turn transistors 144 on, thereby connecting one of theISAMs to one of the transfer buses. Multiplex (mux) lines A through Fare used to isolate sections of the transfer buses during certaintransfer operations. For example, to get the ISAM header from ISAM104(0), the write transfer buses 140 are pre-charged with pre-chargecircuit 141, control line 146 is selectively activated and mux line C(initially activated) is deactivated to isolate the write transfer busesfrom helper flip/flops 148 (HFF). HFFs are known to one skilled in theart, and illustrated in FIG. 7. Mux line A remains turned off to isolateHFFs 150 from the edit buffer control 130 connected thereto. Mux line B,however, is activated so that the contents of the ISAM cell 104(0) canbe captured by HFFs 150. Control line 146 is deactivated to isolate ISAMcell 104(0) from write transfer buses 140. Mux line B is thendeactivated and mux line A is activated so that the edit buffer control130 and CRC logic 128 can access and modify the header,prepend/postpend, and CRC bytes latched in HFFs 150.

As explained above, the 32-bit header and the 32-bit prepend/postpenddata can be modified using the edit buffer control 130. This isaccomplished by selectively changing the state of HFFs 150. A new CRCcode can also be generated for the modified header using CRC generator128 such that the HFFs 150 associated with the 8-bit CRC code areselectively changed using edit buffer control 130. Using HFFs 150provides a fast, low power way of accessing and modifying the state ofthe ISAM bits. After HFFs 150 have been selectively changed, mux line Ais deactivated and mux lines B and C are activated. The contents of HFFs150 are transferred to HFFs 148. HFFs 148 are isolated from the writetransfer bus 140 and the DRAM 102 by deactivating mux lines C and B.

Parity generator 122 has generator circuits 149 which generate a paritybit for every 8 write transfer bus lines. That is, the contents of onebyte of HFFs 148 produce one bit of parity data 151. The 448 bits ofdata transferred from the ISAMs 148 is appended, therefore, with 56parity bits, for a total of 504 bits transferred to DRAM 102.

The 376 data bus lines connected to the ISAMs are used to transfer datato the DRAM through write transfer bus 140 and HFFs 148. The ATM data isnot modified and is therefore directly transferred to HFFs 148 wherecorresponding parity bits are generated. After all parity bits 151 havebeen generated, mux line D is activated and the contents of all 504 HFFsare transferred to a DRAM row address accessed with memory control 136.

Mux line E is activated to perform a read transfer of an ATM cell storedin the DRAM 102 to the OSAMs 106. The contents of the DRAM row accessedis transferred to HFFs 152 and mux line E is then deactivated. Byteparity check 124 has a parity generator 154 which produces a parity bitfor each byte of the 56 bytes of the ATM cell. The parity bits arecompared to the parity bits 151 stored in the DRAM during the writetransfer operation using comparator circuit 156. Any parity errors arethereby identified using Comparator 156 and error status circuit 126.

Mux line F is activated to connect the read transfer buses 158 to theHFFs 152 and the intended OSAM is selectively accessed using controllines 155 and corresponding transistors 157. Output edit register 132(not shown in FIG. 6) is used to modify the contents of the 72 bitscomprised of the header, prepend/postpend and CRC bits. The header andprepend/postpend bytes are modified by using an NOR gate so that eachbit output from the HFFs 152 are NORed with an output from the editbuffer output 134. Similarly, the CRC bits are individually XORed withan output from the edit buffer output 134. A post-DRAM edit is typicallyperformed so that an ATM cell can be broadcast to a plurality oflocations instead of one location. It will also be understood that apost-DRAM edit can be used to change the address of the intendedindividual receiver which was designated prior to storing in the DRAM.

It will be understood that a pre-charge circuit 141, 143, or 145 isincluded with select segments of transfer bus 158 which can be isolatedusing a Mux line. This allows each segment to be pre-charged prior toconnecting to another segment, as described below. It will be furtherunderstood that Mux F and the pre-charge circuit 145 associatedtherewith can be replaced with an inverter and a pre-charge circuit atits input. The input of the inverter would therefore remain high unlessa low signal was present at HFF 152. The output of the inverter wouldthen toggle high from its normally low state.

SAM to HFF Transfer

Referring to FIG. 7, a standard HFF architecture is illustrated. Morespecifically, FIG. 7 shows HFF 150 connected to the edit buffer control130 which is connected to write transfer bus 140(0). Both the true 160and complement 162 lines of the write transfer bus lines 140(0) areshown. Transistors 164 are connected to electrically isolate the writetransfer bus lines 160 and 162 and the HFF true 166 and complement 168data lines. HFF 150 is comprised of two n-type transistors 170 and twop-type transistors 172 having their gates cross-coupled to the true 166and complement 168 data lines. The sources of transistors 170 areconnected to the supply voltage and the drains of transistors 172 areconnected to enable/latch transistor 174. Enable line 176 is connectedto the gate of enable/latch transistor 174. Transistors 178 are used toelectrically connect the HFF 150 to the CRC logic 128. Likewise,Transistors 180 are connected to mux line A and are used to connect HFF150 to output buffer of the control interface and command decode 114.HFF 150 latches the state of the true and complement data lines whenenable line 176 goes high.

Representative voltages and timing of an HFF 150 latch operation of datastored in an ISAM cell header are illustrated in FIG. 8. The voltagesand timing are intended as illustrations and are not to be taken aslimitations. Prior to transferring data from an ISAM to the HFF 150, thewrite transfer bus true 160 and complement 162 lines are bothpre-charged to speed the data latch. The prior-state latched by the HFFin this illustration is such that the true data line 166, is high andthe complement data line 168, is low. The latch enable line 176 togglesto a low state, thereby shutting transistor 174 off. The drains oftransistors 172 are, therefore, floating. Mux line B goes high at 190 toelectrically connect the HFF 150 to the write transfer bus lines 160 and162. The write transfer bus is still pre-charging so that the true line160 remains high and the complement line 162 attempts to remain high asthe complement data line 168 is pulled high. The pre-charge circuit isturned off at 192 and the pass gate, or control line 146, to the SAMcell is also activated at 192 so that the ISAM is connected to the writetransfer bus. In this illustration, the state of the SAM cell is suchthat the true line 160 of the write transfer bus is lower than thecomplement line 162. As a result, the HFF true data line 166 is pulledlow and the HFF complement data line 168 continues to increase until theHFF data lines cross at 194. At the cross-over point 194, the HFFchanges state. That is, the n-type transistor 170 connected to the HFFcomplement data line 168 begins to turn on, thereby continuing to pullthe complement data line higher. After the data lines have crossed over,the enable line 176 is activated so that transistor 174 is turned on.Both data lines 166 and 168 are forced to the power rails at 198 and200, respectively, and the HFF has latched the state present on the ISAMcell. Mux line B is then de-activated at 202 to electrically isolate theHFF from the write transfer bus. Finally, control line 146 isde-activated to isolate the SAM from the write transfer bus and thetransfer bus pre-charge is turned on at 204.

FIG. 8 illustrates the process of latching a state of an ISAM cell whichis opposite of the prior-state of the HFF. It will be understood thatlatching a state which is the same as the prior-state follows a similarprocess, except the HFF never crosses over. Further, after the HFF haslatched the ISAM cell, the HFF can be accessed by the command interfaceand command control 114 or the CRC logic 128, using either mux line A ormux line CRC, respectively. Additional editing and transfers to the DRAMare conducted as explained above.

Conclusion

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

The present invention provides a fast, flexible ATM switch having aninternal transfer circuit which allows portions of an ATM cell to beedited before and after storage on an internal DRAM. The ATM switchprovides a parity generator and parity check comparator circuit tomonitor errors which may be induced during internal transfers. Theinternal transfer circuit comprises helper flip/flops (HFF) such thatATM data can be transferred from an internal ISAM to a HFF, edited, andtransferred directly to another HFF prior to storing in the DRAM. Thetransfer circuit further provides an output edit register comprising ofOR-type gates to edit the ATM data during an output transfer from theDRAM to an OSAM.

What is claimed is:
 1. A method of transferring data in a serial dataswitch comprising a plurality of serial access memories (SAMs) and adynamic random access memory (DRAM) connected to the plurality of SAMs,the method comprising the steps of;loading a serial data cell in one ofthe plurality of SAMs; transferring at least a portion of the serialdata cell to an edit buffer; editing the at least a portion of theserial data cell contained in the edit buffer; transferring the contentsof edit buffer and a remaining portion of the serial data cell to theDRAM; generating a parity code based on the contents of the edit bufferand the remaining portion of the serial data cell; and storing theparity code in the DRAM.
 2. A method of transferring data in anasynchronous transfer mode (ATM) switch comprising a plurality of serialaccess memories (SAMs) and a dynamic random access memory (DRAM)connected to the plurality of SAMs, the method comprising the stepsof;transferring an ATM data cell stored in the DRAM to an error checkcircuit; generating an error code based on the ATM data cell; comparingthe generated error code to a pre-determined error code; editing atleast a portion of the ATM data cell; and transferring the edited ATMdata cell to one of the plurality of SAMs.
 3. A method of transferringdata in a asynchronous transfer mode (ATM) switch comprising a pluralityof serial access memories (SAMs) and a random access memory (RAM)connected to the plurality of SAMs, the method comprising the stepsof;loading an ATM data cell into one of the plurality of SAMs; latchingat least part of the ATM data cell in a first helper flip/flop; editingthe at least part of the ATM data cell; transferring the at least partof the ATM data cell from the first helper flip/flop to a second helperflip/flop; transferring the at least part of the ATM data cell from thesecond helper flip/flop to the DRAM; generating a parity code based onthe at least part of the ATM data cell; and storing the parity code inthe DRAM.